This invention relates generally to fiber optics, and particularly, to a fiber optic alignment system and method.
In a 1993 a paper entitled xe2x80x9cA Fresnel Drag Flow Meter,xe2x80x9d written by R. deCarvalho and J. Blake of Texas AandM University, and W. Sorin of Hewlitt Packard Laboratories, a new method of measuring fluid flow using a fiber optic Sagnac interferometer was described [1]. The principle of operation is that light travels faster with a flowing material than against it. A Sagnac interferometer was used to measure the time difference it took for light to travel around a closed loop, part of which contained the flowing material.
Two problems were found to exist in this flow meter which had to be overcome in order for it to become practical. First, the light has to be taken out of the optical fiber and passed as an optical beam through the flowing material. Then this light has to be refocused back into the optical fiber. The alignment requirements to accomplish this are excessively delicate, especially when the distance between the fiber tips is long. Second, the flow meter was found to give false readings due to time varying turbulence in the flowing material.
One method for achieving automatic alignment is to continuously move a fiber tip in a circular motion and then position the tip so as to minimize the intensity modulation of the light collected by the receiving fiber. However, this method requires measurement of the light collected by the receiving fiber.
Accordingly, a need has arisen for a fiber optic alignment system and method. The present invention provides a fiber optic alignment system and method that addresses shortcomings of prior system and methods.
According to one embodiment of the present invention, a fiber optic alignment system for guiding light beams through a flow and back into the optical fibers is illustrated in FIG. 7a. Light travels both ways through the flow. Because of reciprocity, when the system is well aligned, the light beams traveling in the two directions overlaps in space. To ensure that the counter propagating waves overlap in space, it is sufficient to ensure that they overlap at two distinct points along the z-axis since two points determine a line. This overlap can be accomplished by aperturing the light waves at two points along the path, such as with lenses, placing photodetectors on the apertures, and then controlling the positions of the fiber tips using PZT actuators to center the optical beams within the apertures. An exemplary realization of the photodetector arrangement for detecting the position of the light beam is shown in FIG. 7b. If equal amounts of light falls onto the two photodetectors placed along the Y or X axis, the error signal controlling the position of the fiber in the corresponding plane is zero. FIG. 7b also shows the relative position of the light beam and the photodetectors as well as the corresponding error signal versus the displacement in the X and Y directions in the case where the feedback loop is open. The position of the fiber on the left is controlled by the detectors on the aperture on the right, while the position of the fiber on the right is controlled by the detectors on the aperture on the left.
FIGS. 9 and 10 illustrate a fiber optic alignment system in accordance with the teachings of the present invention used with coherence domain reflectometers. In this example, it is necessary to have light exit an optical fiber, propagate to a mirror, reflect back to the fiber and then re-enter the fiber. As in the system described above, the alignment will be good when the reflected light overlaps the outgoing light. This can be accomplished by positioning the fiber tip such that the outgoing light overlaps the incoming light at one point in the z-axis. The second overlap point is guaranteed to exist at the mirror. Thus, one aperture and detector set which measures the spatial position of the incoming light beam placed near the fiber tip is used to control the position of the fiber tip as shown in FIGS. 9 and 10. In addition to that, the position of the mirror can be controlled keeping it perpendicular to the light beam using PZT transducers.
According to another embodiment of the present invention, a fiber optic alignment system used with a Sagnac interfermoter flow meter where the loop is broken at or near its middle is illustrated in FIG. 11. In this example, errors caused by time dependent changes in the flow rate are alleviated as both optical beams pass the flow at the same time.
Technical advantages of the present invention include providing an automatic fiber optic alignment system to achieve an ultra sensitive flow meter capable of measuring in real time volumetric flow rates as low as a few nanoliters per second. The achieved sensitivity are several orders of magnitude higher than prior volumetric flow sensors. The present invention has applications in a variety of areas such as biology, medicine or chemistry. The present invention also includes an automatic alignment system in order to make the flow meter practical, especially for the applications where air flow is to be monitored.
Other technical advantages will be readily apparent to one skilled in the art from the following figures and descriptions.